Do you have a lot of homeless living in the area?

I don't think homeless would eat foliage and leave the carrot part in the ground.

My money is on a rabbit.

Rabbits. We actually had to shore up the fencing around our carrot bed just last week because I'd left a small gap between two of the panels when I closed it. I thought it would be too small for a bunny to squeeze through, but my now-topless carrots would beg to differ. ;)

Since you're in zone 5 I'm wondering if your seedlings are very small (mine are) so maybe it's rolly polly bugs? They get mine when they're v. small.

I've never had problems with my carrots, but birds would eat the tops of my beets when they were small. I eventually put a netted frame over the sowed seeds, which gets removed when the beet foliage reaches the top of it.